accel/tcg: Fix the comment for CPUTLBEntryFull

When memory region is ram, the lower TARGET_PAGE_BITS is not the
physical section number. Instead, its value is always 0.

Add comment and assert to make it clear.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-Id: <20230901060118.379-1-zhiwei_liu@linux.alibaba.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/cputlb.c

@@ -1193,6 +1193,7 @@ void tlb_set_page_full(CPUState *cpu, int mmu_idx,
     write_flags = read_flags;
     if (is_ram) {
         iotlb = memory_region_get_ram_addr(section->mr) + xlat;
+        assert(!(iotlb & ~TARGET_PAGE_MASK));
         /*
          * Computing is_clean is expensive; avoid all that unless
          * the page is actually writable.
@@ -1255,10 +1256,12 @@ void tlb_set_page_full(CPUState *cpu, int mmu_idx,
 
     /* refill the tlb */
     /*
-     * At this point iotlb contains a physical section number in the lower
-     * TARGET_PAGE_BITS, and either
-     *  + the ram_addr_t of the page base of the target RAM (RAM)
-     *  + the offset within section->mr of the page base (I/O, ROMD)
+     * When memory region is ram, iotlb contains a TARGET_PAGE_BITS
+     * aligned ram_addr_t of the page base of the target RAM.
+     * Otherwise, iotlb contains
+     *  - a physical section number in the lower TARGET_PAGE_BITS
+     *  - the offset within section->mr of the page base (I/O, ROMD) with the
+     *    TARGET_PAGE_BITS masked off.
      * We subtract addr_page (which is page aligned and thus won't
      * disturb the low bits) to give an offset which can be added to the
      * (non-page-aligned) vaddr of the eventual memory access to get

include/exec/cpu-defs.h

@@ -100,12 +100,12 @@
 typedef struct CPUTLBEntryFull {
     /*
      * @xlat_section contains:
-     *  - in the lower TARGET_PAGE_BITS, a physical section number
-     *  - with the lower TARGET_PAGE_BITS masked off, an offset which
-     *    must be added to the virtual address to obtain:
-     *     + the ram_addr_t of the target RAM (if the physical section
-     *       number is PHYS_SECTION_NOTDIRTY or PHYS_SECTION_ROM)
-     *     + the offset within the target MemoryRegion (otherwise)
+     *  - For ram, an offset which must be added to the virtual address
+     *    to obtain the ram_addr_t of the target RAM
+     *  - For other memory regions,
+     *     + in the lower TARGET_PAGE_BITS, the physical section number
+     *     + with the TARGET_PAGE_BITS masked off, the offset within
+     *       the target MemoryRegion
      */
     hwaddr xlat_section;